In recent years, research and development on the positioning techniques have been widely conducted aiming to provide a service according to a location of a client. The services provided have been limited to outdoor location based services (map information, weather information, nearby shop information, and others) using the GPS (Global Positioning System). However, there is increased demand on indoor location based services for places such as hospitals, museums, and shopping mall, (floor guides in hospitals, guides on displayed drawings, shop information, and others). In response to this demand, research and development on a positioning technology utilizing wireless technology such as a WLAN (Wireless Local Area Network) as an easy-to-access wireless environment have been actively conducted (see NPL 1 to 4).
Positioning techniques using indoor WLAN include methods for estimating location based on TOA (Time of Arrival), AOA (Angle of Arrival), and RSS (Received Signal Strength) of a radio wave transmitted/received between an access point and a client. Among these techniques, many technical researches have been conducted on the positioning based on RSS because the configuration of an apparatus is generally simpler than apparatuses used for the other methods.
FIG. 1 illustrates a general configuration of a positioning method based on RSS using WLAN. The system illustrated in FIG. 1 includes positioning server SV1 for positioning, access points AP1 to AP3 (hereafter referred to as an AP) connected to positioning server SV1, and client CL1 connected to an AP.
The operation for positioning according to the configuration is illustrated as follows.
An AP transmits beacons including an ID of the AP in a predetermined cycle and at a predetermined transmission power.
Client CL1 measures the RSS of received beacon from the AP (AP1 to AP3) (AP1-RSS to AP3-RSS), and reports, to a connected AP or a positioning server, the measured RSS as a set with the ID of the AP of the transmission source (such as MAC address) with an ID of the station in a predetermined cycle. In the following description, a case in which client CL1 reports to AP will be described, in particular.
When the AP receives the ID of the AP (MAC address and others) and the RSS information from client CL1, the AP notifies positioning server SV1 connected to the AP of the received information in a predetermined cycle.
Positioning server SV1 positions client CL1 using fingerprints held by positioning server SV1 (AP1-fingerprint to AP3-fingerprint) and RSS information (AP1-RSS to AP3-RSS) reported from client CL1. The positioning method will be described later in detail. Subsequently, the positioning result is notified of the AP connected to client CL1, with the client ID.
When the client ID and the positioning result are received from positioning server SV1, the AP notifies client CL1 of the positioning result of client CL1 connected to the AP. When multiple clients are connected, the series of process described above will be performed for each client. Note that, positioning server SV1 may directly notify client CL1 of the positioning result.
Subsequently, the positioning method using the fingerprint of the RSS will be described in detail. With the positioning method, the processes in an offline phase and an online phase are generally performed. The process in the offline phase is performed in preparation for the positioning, and a map including the RSS which simulates a propagation environment in a positioning range beforehand (hereafter referred to as fingerprint) is created. More specifically, RSS data measured by an operator at positioning candidate points (also referred to as grid points) is set as a reference RSS of the positioning candidate points in the fingerprint. Alternatively, the RSS data at each grid point calculated by using a distance attenuation formula such as Friis equation represented as Equation 1 as the reference RSS of the positioning candidate point of the fingerprint. Upon calculation, the parameters used in the equation are set as representative values, for example.
Using the fingerprint created as described above, a positioning section in positioning server SV1 can calculate an estimated value of the RSS when a signal transmitted from the AP is received by a positioning candidate point by specifying the AP and a positioning candidate point. More specifically, the estimated value of RSS can be extracted from the fingerprint as reference RSS. The fingerprints are created for respective APs (AP1 to AP3) connected to positioning server SV1 (AP1-fingerprint to AP3-fingerprint) (see FIG. 1).RSS=Pt+Gt+Gr+a+c log 10(d)  (Equation 1)Pt: Transmission power, Gt: Transmission antenna gain, Gr: Reception antenna gain,a: Coefficient dependent on frequency used, c: Path loss coefficient,d: Transmission/reception distance
In the process in the online phase, client CL1 is actually positioned, and the fingerprints are updated to follow the actual changing propagation environment.
Various methods have been presented as specific positioning methods. However, as illustrated in Equation 2, many of them are based on estimation using a positioning candidate point which makes a square error of the value of the fingerprint and the RSS to minimum. More specifically, the method used as a base includes calculating the square error between the reference RSS extracted from the fingerprint and the measured RSS and estimating a positioning candidate point which makes the square error minimum as a positioned location.
                    (                  Equation          ⁢                                          ⁢          2                )                                                                      Positioned          ⁢                                          ⁢          location          ⁢                                          ⁢          estimated_i                =                              min            i                    ⁢                      {                                          ∑                k                                  AP                  ⁢                                                                          ⁢                  count                                            ⁢                                                          ⁢                                                                                                                                    RSS                        ref                                            ⁡                                              (                                                  k                          ,                          i                                                )                                                              -                                                                  RSS                        meas                                            ⁡                                              (                        k                        )                                                                                                              2                                      }                                              [        2        ]            RSSref(k, i): Reference RSS of AP#k at grid index iRSSmeas(k): Measured RSS of AP#k
Here, k denotes an index for identifying the AP, i denotes a grid index for identifying a positioning candidate point, and AP#k denotes an access point at index k. The right-hand side of Equation 2 represents a value of grid index i having a minimum value in curly brackets.
There are various methods for updating fingerprints. The following method is the simplest method. More specifically, as illustrated in Equation 3, the measured RSS at the positioned location measured for client CL1 is considered as RSS actually measured reflecting the actual propagation environment, and the reference RSS of the positioning point in the fingerprint is updated. Alternatively, the coefficient for determining the fingerprint (for example, path loss coefficients) is recalculated, and the value of the recalculated path loss coefficient is reflected on other positioning candidate points in the fingerprint, and an estimated value of the signal reception strength at the positioning candidate point is calculated. Subsequently, the recalculated value is registered to the fingerprint as the reference RSS updated. Stated differently, the reference RSS at the positioning candidate point included in the fingerprint is updated based on the RSS actually measured by client CL1 at one position.
                    (                  Equation          ⁢                                          ⁢          3                )                                                                                      ⁢                                                            RSS                ref                            ⁡                              (                                  k                  ,                  estimated_i                                )                                      =                                          RSS                meas                            ⁡                              (                k                )                                              ⁢                                          ⁢                                          ⁢                                    c              updated                        =                                                                                RSS                    ref                                    ⁡                                      (                                          k                      ,                      estimated_i                                        )                                                  -                                  P                  t                                -                                  G                  r                                -                                  G                  t                                -                α                                            log                ⁢                                                                  ⁢                10                ⁢                                  (                                      d                    estimated                                    )                                                              ⁢                                          ⁢                                                    updated_RSS                ref                            ⁢                              (                                  k                  ,                  i                                )                                      =                                          P                t                            +                              G                r                            +                              G                t                            +              α              +                                                c                  updated                                ⁢                log                ⁢                                                                  ⁢                10                ⁢                                  (                                      d                                          k                      ↔                      i                                                        )                                                                                        [        3        ]            RSSref(AP index, grid index): Reference RSSRSSmeas(AP index): Measured RSScupdated: Updated path loss coefficientdestimated: Estimated grid location and distance between APsupdated_RSSref: Updated reference RSS
With a positioning method using the fingerprint described above, positioning with accuracy of an error of approximately few meters may be performed within a fixed positioning range (for example, a hospital, a museum, and a shopping mall).